1. Field of the Invention
The present invention relates to linear beam devices, such as inductive output tubes, used for amplifying a radio frequency (RF) signal. More particularly, the invention relates to an inductive output tube having an extended-interaction output circuit and/or wide-band input circuit.
2. Description of Related Art
It is well known in the art to utilize a linear beam device, such as a klystron amplifier or traveling wave tube amplifier, to generate or amplify a radio frequency (RF) signal. These amplifiers generally include an electron emitting cathode and an anode spaced therefrom. The anode includes a central aperture; by applying a high voltage potential between the cathode and anode, electrons may be drawn from the cathode surface and directed into a high power electron beam that passes through the anode aperture. The electron beam may be directly modulated in density with a grid in front of a cathode as in an inductive output tube (IOT). Alternatively, the electron beam may be modulated indirectly by modulating the velocity of the electrons and allowing fast electrons to overtake slower electrons as in klystrons or travelling wave tubes. In IOTs and klystrons, the RF energy is removed from the electron beam by allowing the electron beam to pass through a discreet, interaction gap in a resonant cavity and allowing the beam to induce a current that in turn creates an electric field that extracts energy from the beam. In the klystron, the velocity modulation of the electrons is also caused by interaction between electrons in the beam and the electric field in discrete gaps in individual cavities. By contrast, in traveling wave tubes, both the electron bunching and energy extraction are distributed and occur along a transmission line that surrounds the electron beam and propagates an RF wave that travels with nearly the same velocity as the electron beam. This is usually called a “slow wave” because it travels at a velocity less than the velocity of light. The transmission line may be comprised of many cavities, with gaps, that store the energy that passes slowly from cavity to cavity through apertures that couple the cavities, or the wave may travel along one or more helical wires and provide an electric field between turns that interacts with the electrons.
Klystron performance may be enhanced with an extended-interaction output circuit (e.g., a slow wave circuit) to provide for larger bandwidth operation. The design of these extended-interaction amplifiers to provide the desired larger bandwidth of frequencies is often based upon a series of cavities through which an electron beam must travel. Likewise by using short lengths of contra-wound helices in the intermediate and output cavities of a klystron instead of using a discreet gap in each, an extended-interaction klystron may provide greater bandwidth in both the electron-bunching and energy extraction functions. A paper describing an extended-interaction klystron using three cavities was written by M. Chodorow and T. Wessel-Berg, “A High-Efficiency Klystron with Distributed Interaction,” IRE Trans. on Electron Devices, pp. 44–55, 1961.
As mentioned briefly above, another type of amplifier, referred to as an inductive output tube (IOT), includes a grid disposed in the inter-electrode region defined between a cathode and an anode. The electron beam may thus be density modulated by applying an RF signal to the grid relative to the cathode or the cathode relative to the grid. After the anode accelerates the density-modulated electron beam, the electron beam propagates across a gap provided downstream within the IOT and RF fields are thereby induced into a cavity coupled to the gap. The RF fields may then be extracted from the output cavity in the form of an amplified and modulated RF signal.
At the end of its travel through the linear beam device, the electron beam is deposited into a collector or beam dump that effectively dissipates the remaining energy of the spent electron beam. The electrons that exit the drift tube of the linear beam device are captured by the collector and returned to the positive terminal of the cathode voltage source. Much of the remaining energy of the electrons is released in the form of heat when the particles strike a stationary element, such as the walls of the collector. This heat loss constitutes an inefficiency of the linear beam device, and as a result, various methods of improving this efficiency have been proposed.
One such method is to operate the collector at a “depressed” potential relative to the body of the linear beam device. In a typical linear beam device, the body of the device is at ground potential, and the cathode potential is negative with respect to the body. The collector voltage is depressed by applying a potential that is between the cathode potential and ground. By operating the collector at a depressed potential, the opposing or decelerating electric field within the collector slows the moving electrons so that they can be collected at reduced velocities. This method increases the electrical efficiency of the linear beam device as well as reducing undesirable heat generation within the collector.
It is also known for the depressed collector to be provided with a plurality of electrodes arranged in sequential stages in a structure referred to as a multi-stage depressed collector. Electrons exiting the drift tube of the linear beam device actually have varying velocities, and as a result, the electrons have varying energy levels. To accommodate the differing electron energy levels, the respective electrode stages have incrementally increasing negative potentials applied thereto with respect to the linear device body, such that an electrode having the highest negative potential is disposed the farthest distance from the interaction structure. This way, electrons having the highest relative energy level will travel the farthest distance into the collector before being collected on a final one of the depressed collector electrodes. Conversely, electrons having the lowest relative energy level will be collected on a first one of the depressed collector electrodes. By providing a plurality of electrodes of different potential levels, each electron can be collected on a corresponding electrode that most closely approximates the electron's particular energy level. Thus, efficient collection of the electrons can be achieved.
As disclosed in U.S. Pat. No. 5,650,751, a substantial improvement in efficiency of an IOT can be realized by operating the device with a multi-stage depressed collector. When the IOT is configured such that beam current passes through the IOT, during modulation of the RF input signal, both the instantaneous DC current and instantaneous collection voltage (weighted by the individual collector currents and averaged over all collectors and over an RF cycle) would go up and down with the level of the modulated RF output voltage, and both would be proportional to the RF output voltage or the square root of the output power. In other words, the instantaneous modulated DC input power would be proportional to the instantaneous modulated RF output power at all power levels, thereby providing very nearly constant efficiency across the operating range of the device with a proper choice of collector electrode voltages. An IOT having a multi-stage, depressed collector is therefore referred to herein as a constant efficiency amplifier (CEA).
For modern UHF radar, larger bandwidth is needed for frequency agility to avoid jamming (e.g., by enemy or malicious forces). Moreover, in the modern UHF radar system, larger bandwidth is needed to accommodate a frequency chirp together with efficient pulse amplitude modulation because this accommodation allows for pulse compression with minimum time side lobes. Accordingly, it would be desirable to provide an IOT and/or a CEA with larger bandwidth and good efficiency.